1. Field of the Invention
The present invention relates to a dual-frequency impedance matching circuit to be inserted between an antenna and an RF circuit in a mobile terminal in order to carry out impedance matching between the antenna and the RF circuit in two arbitrary frequency bands.
2. Description of the Related Art
As cellphone services have become amazingly popular nowadays, there are increasing demands for an even higher degree of mobility and even more versatile telecommunications services. To meet such demands, it is now one of the major technological objects to develop a mobile terminal that has an even smaller size and yet can use multiple telecommunications systems that are currently operating on mutually different frequency bands (such a device is called a “multi-band device”). Quite the same object is shared by an antenna that is an important device operating as a radio wave input/output interface. That is to say, development of an even smaller antenna operating on multiple different frequency bands (which is called a “multi-band antenna”) is awaited.
In actually developing a mobile terminal, however, it is extremely difficult to realize good antenna properties on multiple desired frequency bands just by optimizing the configuration of the antenna. That is why the final frequency adjustment and good impedance matching with an RF circuit often get done by inserting an appropriate matching circuit between the antenna and the RF circuit. Currently, the frequency bands utilized by various cellphone services are in 800-900 MHz range and 1.5-2 GHz range. To realize a multi-band mobile terminal, its antenna should operate on both of these two frequency bands. However, these two frequency bands are so far apart from each other that it is difficult for a normal single-frequency matching circuit to carry out flexible matching and adjustment on both of these two frequency bands. That is why to achieve the object described above, it is preferable to apply a dual-frequency matching circuit that can carry out independent matching on those two frequency bands.
In such a background, some conventional dual-frequency matching circuits that have been adopted so far include a ladder circuit consisting of multiple single-frequency matching circuits and multiple resonant circuits (see Japanese Patent Application Laid-Open Publication No. 2004-242269 (page 18 and FIG. 1) and Japanese Patent Application Laid-Open Publication No. 2006-325153 (page 14 and FIG. 1), for example). FIG. 11 is a circuit block diagram illustrating a circuit configuration for a conventional dual-frequency matching circuit disclosed in Japanese Patent Application Laid-Open Publication No. 2004-242269 (page 18 and FIG. 1).
In FIG. 11, the frequency dependency of impedance (or a single-terminal S parameter) at an output terminal 102 is already known and a load 101 corresponds to an antenna in the situation described above. And the load 101 is connected to a power supply 107 by way of a conventional dual-frequency matching circuit 108 consisting of first, second and third matching circuits 103, 104 and 105. As shown in the block diagrams in FIG. 11, these matching circuits 103, 104 and 105 are parallel or serial resonant circuits, each of which is made up of inductors and capacitors.
The conventional dual-frequency matching circuit 108 shown in FIG. 11 operates as an impedance transformer such that in two desired frequency bands, the circuit 108 transforms the impedance of the load 101 at the output terminal 102 into the impedance value of the power supply 107 at the input terminal 106. That is why in those two frequency bands, the power supplied from the power supply 107 can be passed to the load 101 efficiently without experiencing reflection attenuation.
Consider each of these three matching circuits 103, 104 and 105 as a single circuit block. In that case, the conventional dual-frequency matching circuit 108 shown in FIG. 11 is composed of the two fundamental types of single-frequency matching circuits 121a and 121b shown in FIG. 12 (see Robert E. Collin,—An IEEE Press Classic Reissue—Foundations for Microwave Engineering (Second Edition, IEEE Press Series on Electromagnetic Wave Theory), A John Wiley & Sons, Inc., Publication, ISBN 0-7803-6031-1 (page 323, FIG. 5.17) (hereinafter, Non-Patent Document No. 1), for example), and they are coupled together so as to form a ladder circuit 131 as shown in FIG. 13. FIG. 12 illustrates circuit block diagrams showing the circuit configurations of the two fundamental types of single-frequency matching circuits disclosed in Non-Patent Document No. 1 and FIG. 13 is a circuit block diagram illustrating the circuit configuration of a ladder circuit for use in a conventional dual-frequency matching circuit. It should be noted that the ladder circuit 131 is a circuit configuration that is ordinarily used in various types of filters.
The function of the conventional dual-frequency matching circuit 108 is equivalent to transmitting an RF signal from the input terminal 106 to the load 101 on two desired frequency bands without causing any reflection attenuation. That is why by adopting the ladder circuit 131 shown in FIG. 13, designing a dual-frequency matching circuit is equivalent to designing a band-pass filter, of which the pass bands are those two desired frequency bands. Consequently, in designing the conventional dual-frequency matching circuit 108, the conventional filter designing method can be used effectively, and matching with the input terminal 106 can be done relatively flexibly on two desired frequency bands without depending on the frequency response of the impedance at the load 101. These are advantages of the conventional configuration.
However, the conventional configuration has the following two drawbacks.
Firstly, it is difficult to reduce the loss caused by the dual-frequency matching circuit. To improve the quality of cellphone services, the transmission and receiving properties of mobile terminals must be improved. The transmission and receiving properties are improved mainly by reducing the transmission loss between the antenna and the RF circuit. That is why the loss to be caused by the dual-frequency matching circuit inserted there is preferably as little as possible. The conventional configuration, however, needs too many elements (including inductors and capacitors) and must use a number of resonant circuits, and therefore, is a problem as far as loss reduction is concerned.
Another problem is that it is difficult to stabilize the matching property with respect to the variation in the impedance of the load 101. Normally, when a mobile terminal is used, the user's hand or head comes close to the antenna. That is why the frequency dependency of the impedance for the antenna is affected by how the device is used. For that reason, to ensure stabilized transmission and receiving quality, the matching property must be stabilized with respect to the variation in the impedance of the antenna. However, since the conventional circuit described above includes a lot of resonant circuits, of which the electrical properties (represented by a two-terminal S parameter) vary steeply with the frequency, the matching property is easily affected by the variation in the impedance of the load 101. Furthermore, in the ladder circuit 131, the impedance is transformed in each single-frequency matching circuit 121a, 121b (see FIG. 12) and the ladder circuit is composed of a number of such single-frequency matching circuits. That is why the ladder circuit itself is sensitive to the variation in the impedance of the load 101. In view of these considerations, the conventional configuration described above is still to be improved in terms of stability, too.
In order to overcome the problems described above, the present invention has an object of providing a dual-frequency matching circuit that causes little loss and that achieves high stability with respect to a variation in the impedance of a load.